The present invention relates to a method for statistical multiplexing of ATM Links.
Two types of connection are generally defined for connections via which information is transmitted using an asynchronous transfer mode (ATM). A distinction is thus drawn between, connections by means of which information is transmitted at a constant bit rate (CBR) and connections via which information is transmitted at a variable bit rate (VBR). In this case, the information is transmitted in ATM cells. Particularly in the case of variable bit rate transmissions, the ATM cells are transmitted in an irregular time sequence, as a result of which so-called “bursts” occur. This means that the cells are transmitted at an increased level in a short time interval, while no cells are transmitted in the remaining time. A range of transmission parameters have been defined to describe these connections. These include, for example, the peak cell rate (PCR). This is an upper limit for the number of cells which can be transmitted by a transmitting device per second.
In general, when setting up an ATM link, the transmitting device must report previously defined parameters to a higher-level control device (call acceptance control). This is necessary to ensure the quality of the connection for all subscribers (quality of service). If, for example, too many cells are transmitted, and the transmission capacity is thus exceeded, too many cells would have to be rejected. However, this must always be avoided since this is inevitably linked with a loss of information. Standardization committees, for example, have produced a requirement for a cell loss probability of 10−10 for a link, in this case. For this reason, even while the connection is being set up, a calculation is carried out to determine whether this new connection can be added to already existing connections. If the transmission capacity is already exhausted, the requesting connection is rejected.
These processes are handled in the higher-level control device by an algorithm which checks the parameters received from the transmitting device. Furthermore, these parameters are compared with already calculated parameters relating to the current load on the connecting line. These comparisons are then used as the basis to decide whether the new call request can be accepted, and this call can also be approved. The already-mentioned peak cell rate (PCR) is used as a parameter. In addition, for a connection with variable bit rate, the transmitting device reports a sustainable cell rate (SCR) to the control device. This is the upper limit for a mean cell rate at which the cells are transmitted while the connection exists. As further parameters, the control device recognizes the maximum possible transmission capacity of the connecting line (link cell rate, C) and the maximum possible load on the connecting line (Po). The former is effectively an equipment constant for the connecting line, while the latter defines a variable which is used to indicate the maximum permissible total cell rate on the connecting line. As a rule, this is 95% of the maximum possible transmission capacity of the connecting line. This parameter is then used to decide whether new call requests can or cannot be accepted.
In the prior art, a number of methods have been developed for handling these processes. The peak cell rate reservation algorithm should be mentioned here, as a simple method. In this case, an n-th connection is approved only if the following expression is satisfied for the (n−1) already existing connections plus the n-th connection:                                           ∑                          i              =              1                        n                    ⁢                                          ⁢                      PCR            i                          ≤                              p            0                    ·          C                                    (        a        )            
If this condition is not satisfied, the call request is refused.
The sigma rule algorithm should also be mentioned as another known method. This method is described in the document “E. Wallmayer, ‘Connection acceptance algorithm for ATM-Networks based on mean and peak bit rates’, International Journal of Digital and Analog Communication Systems, Vol. 3, pp. 143 to 153, 1990”. In this case, this known method is a further development of the peak cell rate reservation algorithm. In addition to the condition (a), another condition (b) must also be satisfied here:                                                         ∑                                                VC                  i                                ⁢                                                                  ⁢                ɛ                ⁢                                                                  ⁢                Class                ⁢                                                                  ⁢                S                                                                                  ⁢                                                  ⁢                          SCR              i                                +                      q            ⁢                                                  ⁢                                          (                                  c                  ,                                      Class                    ⁢                                                                                  ⁢                    S                                                  )                            ·                                                (                                                            ∑                                                                        VC                          i                                                ⁢                                                                                                  ⁢                        ɛ                        ⁢                                                                                                  ⁢                        Class                        ⁢                                                                                                  ⁢                        S                                                                                                                                  ⁢                                                                                  ⁢                                                                  SCR                        i                                            ·                                              (                                                                              PCR                            i                                                    -                                                      SCR                            i                                                                          )                                                                              )                                                  1                  /                  2                                                                    ≤                                            P              0                        ·            C                    -                                    ∑                                                VC                  i                                ⁢                                                                  ⁢                ɛ                ⁢                                                                  ⁢                Class                ⁢                                                                  ⁢                P                                                                                  ⁢                                                  ⁢                          PCR              i                                                          (        b        )            where c=po·C−Σ PCRi, the free capacity for Class S.
It can be seen from condition (b) that the existing connections are split into 2 classes here. At the start of the process of setting up a connection, the sigma rule algorithm therefore has to decide which of two classes, namely a Class S or a Class P, the possibly newly incoming ATM link must be allocated to.
Class S is used for all virtual connections for which statistical multiplexing in accordance with the sigma rule algorithm would result in a clear improvement over the peak cell rate reservation algorithm. As a rule, these are low bit-rate connections. As a criterion for connections of this type, the peak cell rate and the sustainable cell rate of all the connections to be multiplexed statistically must satisfy the following condition:PCR/C<0.03 and (0.1≦SCR/PCR≦0.5)
Class P is used for all the other virtual connections. These include, in particular, those connections at a constant bit rate. This also includes all the connections for which the parameters SCR and PCR are very close to one another—or are very far apart from one another, or which already have a high peak cell rate PCR. One criterion for this is a peak cell rate that is greater than 3% of the maximum possible transmission capacity of the connecting line.
Furthermore, condition (b) includes a factor q. This factor is dependent not only on the Class S but also the free capacity c of Class S. For a defined Class S, the q(c) values must be calculated using a complex program. For simplicity with regard to dynamic aspects, the dependency on the variable c is estimated by a hyperbolic function q(c)=q1+q2/c).
Thus, in this prior art, an n-th virtual connection VCn with a defined peak cell rate PCRn and a sustainable cell rate SCRn is approved for (n−1) already existing virtual connections VCi with the parameters SCRi as well as PCRi (1≦i≦n−1) on a connecting line, provided the conditions (a) or (b) are satisfied.
Using condition (a), a check is carried out to determine whether the sum of the peak cell rates of all n connections on the connecting line is less than or equal to the maximum possible transmission capacity on the connecting line. If this is the case, then the n-th virtual connection can be accepted, and the question in condition (b) is superfluous. If this is not the case, then condition (b) is used to check whether the upper estimate of the mean value of the sum of the peak cell rates of all the connections in Class S, together with a cell rate which is calculated from the burst probability of all the connections in Class S, is less than or equal to the cell rate which is currently available for Class S connections. If this is the case, then the n-th virtual connection is accepted, and if not it is rejected.
A disadvantage with this prior art is that, when the sigma rule algorithm is used, the maximum transmission capacity on the transmission line is not completely exhausted.
European Patent Application EP 0 673 138 A2 discloses a method for approval of ATM links. According to this document, the connections to be accepted are subdivided into traffic classes, and a calculation is carried out to determine whether the new connection can be accepted, in terms of its bandwidth. Thus, however, in this case as well, the maximum transmission capacity on the transmission line is not completely exhausted.
European Patent Application EP 0 596 624 A2 likewise discloses a method for approval of ATM links. Once again, the connections to be accepted are subdivided into traffic classes. In this case as well, this document does not address fine control for the purpose of completely exhausting the maximum transmission capacity on the transmission line.